Many video display systems show beat patterns when the video data contains specific display patterns. These beat patterns show up on a display as bands, streaks, or other forms. The beat pattern frequencies are not normally an integer multiple of half of the video horizontal scanning frequency, and hence the beat patterns tend to manifest themselves as moving patterns. Until now, it has not been known exactly what caused the beat patterns, and very little was known about how to eliminate them.
Existing attempts to address beat patterns typically seek to alter or tune the frequencies of the particular subsystems to minimize the symptoms of the beat patterns. For instance, one method sometimes used is to force the frequency of the beat pattern to some relationship of the horizontal and vertical scanning frequencies so that the pattern is non-moving. This method does not eliminate the beat pattern but rather hides it from the human eye. This method takes advantage of the fact that the human eye cannot easily detect non-moving objects, especially objects with fuzzy outlines. However, even if the beat pattern is not moving and nearly invisible to the human eye, the diminished video quality is still present.
Another method sometimes used is the canceling method. With this method, inverted beat pattern attributes (such as intensity) are introduced on consecutive horizontal scan lines so the pattern seems to “cancel out” its appearance.
Still another method sometimes used is the resizing method. With this method, one or more of the system frequencies is changed such that the physical size of the interfacing bands is ether very small or very large to the viewer. If the beat frequency were set to a much higher frequency than the horizontal scanning frequency, then the “bands” would become tiny, and hence less visible to the viewer. Similarly, if the beat frequency were set sufficiently low (e.g., 0Hz), then only one band with uniform intensity would appear on the screen. From the viewer's perspective, the interfering patterns would no longer exist.
Yet another method sometimes used is the blurring method. This technique involves sweeping one or more of the system frequencies in a pseudo random manner. This technique results in a blurred beat pattern which is less visible to the viewer.
Unfortunately, these existing attempts to solve the beat pattern problem are inadequate because they do not address the problem but rather only try to mask its symptoms. The result of each of these methods is merely a beat pattern which may be less visible to the viewer, but which still results in diminished video quality. Accordingly, both the actual problem causing beat patterns and an acceptable solution to that problem have eluded those skilled in the art.